Caption: Acute appendicitis. Light micrograph through a section of an appendix showing early-stage acute appendicitis. Here, the epithelial lining of the appendix (dark purple, top left) has become ulcerated (deep pink region, lower left) and is secreting an inflammatory exudate (thick fluid, along left edge). White blood cells (small, dark purple) are present in high numbers. As the disease progresses, the inflammation will spread until until the exudate is present on the external surface (peritoneum) of the appendix. Inflammation of the peritoneum (peritonitis) is serious and often requires emergency surgery.